As the climate warms, hydrological processes become more complex, making the development of accurate digital twins of the terrestrial water cycle increasingly crucial for predicting water-related hazards and managing water resources effectively. In this study, we investigate the use of satellite observations of the Earth’s time-variable gravity fields from the Gravity Recovery and Climate Experiment (GRACE) and its Follow-On mission (GRACE-FO), to estimate Terrestrial Water Storage (TWS) and to constrain water storage representation of large-scale hydrological models within Europe. The implementation follows a multivariate state-space Bayesian model-data fusion to merge a priori 0.1° resolution water storage estimates with the TWS observed by GRACE/GRACE-FO. The estimated groundwater storage will be validated against groundwater network within Europe and the GNSS measurements. We also assess the feasibility of the (Interferometric) Synthetic Aperture Radar (InSAR) and SAR techniques for downscaling water storage estimates in selected regions in Europe
